U.S. Pat. No. 4,308,557 of C. B. Dieterich entitled "VIDEO DISC SYSTEM" which issued Dec. 29, 1981 incorporated herein by reference, describes video disc recording and playback apparatus wherein video fields on a disc are identified by digital information recorded during a selected line of the vertical interval during each field of the recorded video signal. This "auxillary" digital information, commonly known as the system "DAXI" code, is utilized to control a number of video disc player operating functions during playback of the disc such as calculation and display of elapsed playing time, lifting the pick-up stylus at the end of the program material, detection and correction of locked grooves, etc.
As proposed by Dieterich, the recorded DAXI code includes, in the order named, a Barker start code sequence, an error detection check code and a plurality of information bits which include a field number corresponding to the recorded video field. The DAXI code format desirably provides improvements in noise immunity and simplifies the decoding hardware requirements for the player.
Further improvements for decoding the DAXI data of the Dieterich format are described by Christopher in U.S. Pat. No. 4,309,721 entitled "ERROR CODING FOR VIDEO DISC SYSTEM" which issued Jan. 5, 1981. In Christopher's system the error code portion of the DAXI data is chosen so that the error code check register in the video disc player begins with the system start code in the register and, if no errors are detected after the full data message is received, also ends with the start code in the check register. This desirably simplifies the DAXI decoding logic in the video disc player.
Other U.S. Patents relating to use of DAXI code in a video disc player are U.S. Pat. No. 4,307,418 entitled "VIDEO DISC PLAYER SYSTEM FOR CORRELATING STYLUS POSITION WITH INFORMATION PREVIOUSLY DETECTED FROM DISC" which issued Jan. 26, 1982, and U.S. Pat. No. 4,313,134 entitled "TRACK ERROR CORRECTION SYSTEM AS FOR VIDEO DISC PLAYER" which issued Jan. 26, 1982, to Rustman et al.
Video disc records employing the DAXI encoding format are commercially available, for example, from RCA Corporation and CBS, Incorporated. In such records, chrominance information is recorded in the "buried subcarrier" (BSC) format proposed by D. Pritchard in U.S. Pat. No. 3,872,498. The DAXI code is recorded by pulse code modulation (PCM) of the luminance signal level during line 17 of odd fields and line 280 of even fields. The DAXI data comprises a 77 bit PCM word synchronized with the "buried" color subcarrier frequency (about 1.53 MHz for NTSC compatible players) to facilitate subsequent detection in the player by comb filtering. Each DAXI word comprises a 13 bit start code (a Barker sequence for data framing) followed by a 13 bit CRC (cyclic redundancy check) error check code and ending with a 51 bit information code. Of the 51 bits in the information code, 6 provide a record band number, 18 provide a video field identification number and the remaining 27 are currently not assigned but are included to provide information capacity for future expansion or other uses of the DAXI code.
In video disc players for use with DAXI encoded discs, the PCM signal is obtained from the subtractive output tap of a 1-H delay comb filter in the video signal path. Each line of DAXI code is preceeded by a blank video line. As a result of the subtraction of the current and previous video lines, the resultant PCM signal is "self-referenced" and is therefore relatively unaffected by D.C. drift. This simplifies subsequent PCM detection and reduces potential bit errors which might other wise occur without such drift compensation. Examples of video disc players wherein DAXI information is comb filtered prior to PCM detection are given in U.S. Pat. No. 4,275,416 of Dieterich entitled "PCM DETECTOR" and U.S. Pat. No. 4,278,992 of Christopher entitled "PCM DETECTOR FOR VIDEO REPRODUCER APPARATUS".